[sdiy] Shielding -

[harrybissell]

Magnus Danielson wrote: <snipped>

> > >Of course, this is electrostatic shielding, magnetic shielding would be much
> > >more expensive.
> >
> > What options are available, if you want to provide magnetic shielding as
> > well as electrostatic shielding?
>
> Actually, what people have missed is that you dampen of magnetic fields with
> metalic sheets too. The field will induce a current in the metal which will
> cause a inverse field and the vector sum will decrease. Since the resistance
> in the metal is more than zero, there is losses, so naturally will the effect
> not be total, and due to the laws of physics it will be frequency dependent.
>
> Well, you can get perfect cancelation if you use a super-conductor, but not for
> arbitrary strengths of the field since the superconductor saturates in current
> density which you do not want to do. I've personally use the reverse-induction
> field in the form of the Meisner-effect to keep the superconductor to "fly" over
> a magnet. Cool stuff. It was a YBaCuO-superconductor cooled with liquid
> nitrogene.
>
> <SNAP BACK>
>
> Oh... those memories...
>
> Anyway, the damping becomes lower with lower frequency, so it shields for
> higher frequencies. In coax cables it is a combination of the shield directly
> and symmetry and cancelation which causes the full effect. For shielded twisted
> pair will the twisted pair become more effective with lower frequencies, so a
> properly shielded twisted cable with proper diffrential drive and receiver is
> actually a pretty good system.
>
> A shielded room works well for most of the noise we receive. 50 Hz and the like
> isn't well shielded, but FM radios, etc. should be really well blocked.

Most of the 50/60Hz in a musical environment comes from the power supply itself...

Keeping building wiring WELL away from the area works well. Careful attention to the

grounding etc... is good as well. Avoid Fluorescent lights, light dimmers.
Incandescents
using a Variac for dimming are good as well.

I KNOW this stuff... because MY studio is in a 100 year old house with Knob and Tube

wiring (met the National Electrical Code when installed in 1915).  The feed and
return
conductors are 16" apart (adjacent wall studs)... which probably worked FINE when
the only use was resistive (light bulbs)... but with a reactive load it makes a very
fine antenna
for radiating magnetic fields.  Worse still... I use diode light dimmers
(half-wave)...   :^P

Still... the studio is fairly quiet, except for single coil guitars. Being in a BAD
room can be a good
thing... it makes you design for worst-case scenarios from the start. Then you are
not likely to find a worse room in the field (I haven't yet...)

I'm using Balanced Power (60-0-60) in the USA. You Euro-folk already have balanced
power.
I still get a benefit from the isolation / shielding of the transformer...

...and I keep Larry Hendry away from my house. The Ground of the studio is not
bonded to the House ground. So much for NEC eh, Larry ???

> However, there are simple means to make sure that RF doesn't enter into devices.
> You can do minor wonders with just capacitors and some planning.
>
> > If the room is underground, would that help much?
>
> Yes, some. The ground around the house will also attenuate signals in a similar
> sense. However, it varies with the weather... dry ground probably isn't as good
> as wet ground.
>
> > I think it's a bit odd that electrostatic shielding of the room would help
> > so much, if the equipment inside the room is built with good shielding to
> > begin with. Maybe that was the problem--the gear inside the room wasn't
> > really designed with excellent shielding?
>
> Hey! Where talking audio-devices here... I *rarely* see anything which looks
> like serious RF-protection measures in audio gear. I think building shielded
> rooms for studios is greatly overdoing it. If equipment is sensitive to it, why
> not fix it? All EMI/EMC protection done after a device have been designed always
> becomes expensive. Doing a shielded room is overdoing it. For a DIY-er you can
> do alot by fairly simple means if you want to.
>
> * IEC-outlet integrated with a line filter with a real shield around it.
>
> * Metalic case with good conduction between all surfaces and protective ground.
>   Especially along the edges should conduction be good along the full edge.
>
> * Ceramic caps to shield/case for all ingoing and outgoing electrical signals.
>   This shorts common mode signal into shield.
>
> * A cap between hot (+) and cold (-) of diffrential signals.
>   This shorts diffrential mode RF signals.
>
> * A series shunt-inductor will also help for signals. For diffrential signals
>   a double-inductor (really a 1:1 transformer around a circular core turned
>   90 degrees wrongly... the polarity mark of both coils in the same direction)
>   is the best choice. You usually see clamps on monitor cables, PSU cables of
>   portables etc., they are exactly this - put applied on the wrong side of the
>   contact IMHO.
>
> * For these caps, keep absolutely minimal lead-distance since they are to handle
>   RF and not audio, so any series-induction will reduce the effect. I would
>   start of with a 100 pF cap. Large caps actually make things worse most of the
>   time contruary to popular beleif (the exception is in resonant curcuits where
>   you probably want to increase the loss of that resonance to get started).
>
> Some of these measures can be retrofitted into existing devices. However like
> always, doing it after a box is designed makes it more expensive than
> integrating it into the design.
>
> A word of warning thought. Do not apply the caps to shield on the PCB end of
> cables from the contact, then you inserted too much induction for them to be
> effective, right at the contact is where they should be. If you contacts is
> PCB mounter, just toss the caps on the solder-side, and trim down the leads to
> become minimal - big loops isn't doing it the right way.
>

Right... no big loops. Is 16" centers considered a 'big' loop ???   :^P

H^) harry

'with a history of doing EMC stuff the hard way'